1. Field of the Invention
The present invention relates to a throttle valve which is relatively thick and has a round circumferential edge. More particularly, it relates to a throttle valve disposed within the intake bore of an internal combustion engine, which valve is adapted to reduce the degree of air flow change at small opening angles of the valve as in the idling or low-speed operation of the engine so as to reduce the change of air flow caused by the change of the valve opening.
2. Description of the Prior Art
Usually, a throttle valve of a butterfly type as shown in FIG. 1 is disposed within the intake bore of an internal combustion engine. Fuel from a main fuel system or a slow fuel system is mixed with air, the flow of which depends upon the opening of the throttle valve, and an optimum mixture of fuel and air is supplied to the combustion chamber of the engine. When the conventional throttle valve is fully closed, the clearance L between the valve 1 and the intake bore 2 is usually about 0.005 mm to 0.03 mm which is the smallest possible clearance that prevents the valve from sticking. The full closed angle .theta. of the throttle valve is in a range of 5.degree. to 20.degree., and the thickness t, thereof is 1.0 mm to 1.5 mm. According to such a construction, air flow is zero when the valve is fully closed and it sharply increases therefrom as the valve is opened, as shown by the dot-dash curve in FIG. 3, by the effect of the clearance between the valve 1 and the intake bore 2, the initial angle of the valve, etc. This means that the relationship between the change of the valve opening and the change of air flow is critical, and an error in the valve opening exercises a great influence upon an error in air flow. Therefore, a compensating means is used in addition to the slow fuel system in order to obtain an approximation to a desired air-fuel ratio at small opening angles of the valve. However, such an effort has not yet obviated the aforesaid critical relationship.